The present invention relates to a method and apparatus for feeding particulate material into a melting chamber, and more particularly to a method and apparatus for feeding particles of glass into a crucible or muffle wherein the particles are heated to or above melting temperature and from which molten material is extruded in the form of filaments. Still more particularly, the invention relates to improvements in a method and apparatus for continuous simultaneous extrusion of a plurality of glass filaments by resorting to an elongated crucible with one or more rows of extrusion nozzles.
Apparatus for the making of glass filaments normally employ a crucible or muffle defining a melting chamber for a charge of molten glass and having one or more orifices serving to discharge streamlets of molten glass which harden immediately or shortly after they leave the orifices of the nozzles to be thereby converted into elongated filaments which are thereupon collected, grouped and/or otherwise manipulated, depending on their intended use. The crucible consists of a highly heat-resistant metallic, refractory or other substance, and its chamber must receive fresh material (glass in solid state) in order to insure that the upper surface of the molten charge in its chamber does not descend below a minimum permissible level. The nozzles for extrusion of filaments normally form part of a spinneret which is installed in the crucible to constitute the bottom wall or base plate of the melting chamber. The spinneret may be formed with one or more rows of nozzles, and the filaments issuing from the orifices of such nozzles are engaged and convoluted by a rapidly moving rotary body, such as a drum, roller, spool or the like. The rotary body also serves as a means for attenuating the filaments to a desired thickness. In the manufacture of filaments of finite length, the filaments are caused to contact the periphery of a rotary drum whereon they remain parallel to each other and are convoluted on the periphery of the drum to form a hollow cylindrical layer of desired thickness (glass wool). If the apparatus is used for the making of relatively short staple fibers, the drum cooperates with a stripper or doctor blade which severs the filaments before they can form complete convolutions around the periphery of the drum. It is also known to assemble the filaments which issue from the nozzles of the spinneret into two or more tows or multi-filament yarns which are thereupon convoluted onto bobbins or spools to form threads known as glass silk.
For the making of glass wool or staple fibers, the apparatus of presently known design normally employ a relatively large drum having a diameter of up to or in excess of 1,000 millimeters and an axial length which equals or closely approximates its diameter. Such large drum cooperates with a crucible having a spinneret whose length equals or approximates the axial length of the drum and which is formed with one or more rows of nozzles whereby each row comprises a substantial number of discrete nozzles. Thus, the length of the melting chamber in the crucible can reach or even exceeds 1 meter. This presents serious problems as concerns the homogeneousness of molten charge in the crucible. A satisfactory consistency of molten charge can be insured only if the volume or mass of molten charge is rather small, i.e., if the elongated chamber is relatively narrow and the height of molten charge therein is rather small. Otherwise, the heating means in or for the crucible cannot guarantee a satisfactory heating of each and every region of the molten charge so that, once the mass of such charge exceeds a relatively small maximum permissible value, it develops streaks not only in the region of its upper surface but also in its interior to thus affect the quality of filaments which issue from the nozzles of the relatively long spinneret. Additional problems arise due to the fact that the crucible must receive fresh material at the rate at which molten material is being extruded through the nozzles, i.e., freshly admitted particles of glass must melt and blend into the previously molten material at the rate at which the charge is being depleted when the apparatus is in use. As a rule, fresh material is fed into the melting chamber by gravity so that it dips into and is submerged in the charge to thereby melt and replenish the contents of the crucible.
An essential requisite for the extrusion of continuous glass filaments is that the viscosity of molten charge in the region just above the nozzles of the spinneret remain constant. Thus, the charge above the spinneret must be refined or homogenized to a very high degree so as to be free of streaks, air bubbles or the like. This cannot be readily achieved in view of the desirability of a relatively small melting chamber, i.e., a melting chamber which does not contain a large quantity of molten material. Were the melting chamber designed to receive a relatively large quantity of molten glass (this is not desirable on the aforementioned ground that the heating means cannot maintain all zones of a large charge at a given temperature), the quantity of admitted particulate material relative to the quantity of molten material would be rather small so that the viscosity of molten charge would change very little or not at all, especially if the particulate material (solid glass particles) were admitted into the crucible at a substantially constant rate. Consequently, the quality of filaments and the condition of molten glass in a crucible having a relatively small melting chamber depends to a very large extent on the rate at which fresh material is being fed into the crucible.
Many presently known apparatus for the making of glass filaments use glass particles in the form of pellets, i.e., particles of spherical or substantially spherical shape. A drawback of such apparatus is that the manufacture of glass pellets is an expensive procedure which contributes significantly to the cost of the ultimate produce. Therefore, the manufacturers of glass filaments prefer to employ particles of other than spherical shape, especially particles in the form of small tablets or groups of tablets resembling portions of chocolate bars. Such tablets can be produced with a reasonably high degree of uniformity, i.e., their weight or mass deviates rather negligibly from a preselected value so that, by feeding such tablets into the crucible at a substantially constant rate, one can insure that the upper surface of molten charge in the chamber of the crucible will fluctuate only within a permissible range. However, the particles in the form of tablets or cakes exhibit the drawback that they are normally obtained by breaking up a relatively large bar or plate into a large number of similarly configurated fragments whereby portions of such fragments exhibit ragged edges, recesses, protuberances and/or other unevennesses which interfere with uniform feeding of tablets into the melting chamber.
The feeding of spherical and/or tablet-shaped glass particles into relatively small crucibles, and especially into relatively small crucibles having round melting chambers (such crucibles are often used in the making of glass silk) presents no serious problems because the particles can be caused to enter the charge at the center of the melting chamber. Such central feeding does not affect the uniformity of viscosity of the charge in the region of the nozzles. However, the situation is aggravated when the crucible has a long melting chamber because the admission of particles into one and the same region will invariably cause pronounced changes in viscosity, especially since the capacity of the crucible is relatively small. It was already proposed to employ in such apparatus a feeding device with several adjacent chutes for admission of glass particles into different portions of an elongated melting chamber. The particles are fed by gravity, for example, in a manner as disclosed in German Pat. No. 1,596,667. It has been found that such apparatus operate properly only if the particles resemble or constitute spheres. It has also been found that heat which is being radiated by the crucible and the charge therein melts the particles at the discharge ends of discrete chutes so that the particles soften and adhere to their chutes and block the progress of the next-following particles. Such softening of particles and their adherence to the chutes is observable even if the particles are perfect spheres. Removal of softened particles from the chutes is a time-consuming operation, especially since the attendants must be shielded from heat.